1. Field of the Invention
This invention relates to a method of monitoring the progress and pattern of a combustion or flame front being advanced through a combustible subterranean carbonaceous stratum, and thereafter controlling the progress of said flame front. In particular, this invention relates to a method of monitoring both the vertical and lateral movement of an underground flame front and injecting gases into the vicinity of the combustion area to control the flame front. More particularly, this invention relates to a method of monitoring the pattern and spatial orientation of a flame front during in situ retorting of oil shale and injecting and controlling the flow of fuel or flue gases into the retort to control the speed, extent and uniformity of the flame front in the retort.
2. Related Applications
Subject matter disclosed in this application is also disclosed in commonly assigned U.S. applications Ser. No. 925,064, now U.S. Pat. No. 4,167,213, Ser. No. 925,065, now U.S. Pat. No. 4,184,548, Ser. No. 925,176, now U.S. Pat. No. 4,210,867, Ser. No. 925,177, now U.S. Pat. No. 4,210,868, and Ser. No. 925,178, now U.S. Pat. No. 4,194,026; all of said applications filed concurrently herewith and expressly incorporated herein by reference.
3. Description of the Prior Art
The term oil shale refers to sedimentary deposits containing organic materials which can be converted to shale oil. Oil shale contains an organic material called kerogen which is a solid carbonaceous material from which shale oil can be retorted. Upon heating oil shale to a sufficient temperature, kerogen is decomposed and a liquid product is formed.
Oil shale can be found in various places throughout the world, especially in the United States in Colorado, Utah and Wyoming. Some especially important deposits can be found in the Green River formation in Piceance Basin, Garfield and Rio Blanco counties, and northwestern Colorado.
Oil shale can be retorted to form a hydrocarbon liquid either by in situ or surface retorting. In surface retorting, oil shale is mined from the ground, brought to the surface, and placed in vessels where it is contacted with hot retorting gases. The hot retorting gases cause shale oil to be freed from the rock. Spent retorted oil shale which has been depleted in kerogen is removed from the reactor and discarded.
In situ combustion techniques are being applied to shale, tar sands, Athabasca sand and other strata in virgin state, to coal veins by fracturing, and to strata partially depleted by primary and even secondary and tertiary recovery methods.
In situ retorting oil shale generally comprises forming a retort or retorting area underground, preferably within the oil shale zone. The retorting zone is formed by mining an access tunnel to or near the retorting zone and then removing a portion of the oil shale deposit by conventional mining techniques. About 5 to about 40 percent, preferably about 15 to about 25 percent, of the oil shale in the retorting area is removed to provide void space in the retorting area. The oil shale in the retorting area is then rubblized by well-known mining techniques to provide a retort containing rubblized shale for retorting.
A common method for forming the underground retort is to undercut the deposit to be retorted and remove a portion of the deposit to provide void space. Explosives are then placed in the overlying or surrounding oil shale. These explosives are used to rubblize the shale and preferably form rubble with uniform particle size. Some of the techniques used for forming the undercut area and the rubblized area are room and pillar mining, sublevel caving, and the like.
After the underground retort is formed, the pile of rubblized shale is subjected to retorting. Hot retorting gases are passed through the rubblized shale to effectively form and remove liquid hydrocarbon from the oil shale. This is commonly done by passing a retorting gas such as air or air mixed with steam and/or hydrocarbons through the deposit. Most commonly, air is pumped into one end of the retort and a fire or flame front initiated. This flame front is then passed slowly through the rubblized deposit to effect the retorting. Not only is shale oil effectively produced, but also a mixture of off-gases from the retorting is also formed. These gases contain carbon monoxide, ammonia, carbon dioxide, hydrogen sulfide, carbonyl sulfide, and oxides of sulfur and nitrogen. Generally a mixture of off-gases, water and shale oil are recovered from the retort. This mixture undergoes preliminary separation (commonly by gravity) to separate the gases, the liquid oil, and the liquid water. The off-gases commonly also contain entrained dust and hydrocarbons, some of which are liquid or liquefiable under moderate pressure. The off-gases commonly have a very low heat content, generally less than about 100 to about 150 BTU per cubic foot.
One problem attending shale oil production in in situ retorts is that the flame front may "channel" through more combustible portions of the rubble faster than others. The resulting nonuniform or uneven passage of the flame can leave considerable portions of the rubblized volume bypassed and unproductive. Such channeling can result from nonuniform size and density distributions in the rubblized shale. If the shape of the flame front can be defined or packing variations detected within the retort, then channeling and its effects can be mitigated by controlling the air injection rate and oxygen content into various segments of the retort, or by secondary rubblization if regions of poor density can be mapped.
A variety of prior art techniques have been established for determining the position and progress of underground combustion. Various methods have also been employed to control the progress of underground combustion.
The techniques employed to monitor the position and progress of underground combustion range from indirect theoretical mathematical formulations on the one hand, to rather simplistic direct measurements that can be done at the combustion site on the other. One method relates the pressure fall-off observed at the bottom of the well hole of either injected liquid or effluent gases to the approach of the flame front. A second method employs infrared imaging to detect thermal energy from subsurface heat to identify hot portions of the surface terrain. Simple periodic measurements of the elevation of the ground at a variety of points above the path of the combustion front are used to identify portions of the ground that exhibit a slight rise in elevation due to the presence of a combustion front directly under the elevated point.
Fuel packs have been used in which separate masses of gas-forming materials are spaced at predetermined distances. The release of the identifiable gases at spaced intervals can be related to the position of the combustion front in a particular fuel pack. Another method involves an analysis of effluent gases and a correlation between concentration levels of certain gases to the efficiency of the underground combustion. A similar sample-and-analysis technique involves monitoring various physical properties of the fluids which enter a production well for a change in any two properties, thereby signaling the proximity of a combustion front. Thermocouples have also been used to monitor temperature changes of the overburden to ascertain the position of the flame front. This method can also be employed in a down-hole version. Self-potential profiling has been used to detect self-potential voltages generated by the underground combustion. Finally, high frequency electromagnetic probing can be used to observe the progress of the flame front by its effect upon reflected radiofrequency waves.
The methods taught by the prior art are, in general, directed towards either (1) detecting lateral movement of a flame front, or (2) the vertical movement of a flame front, but not both. In addition, even those methods which are capable of detecting the directional movement and location of the front do not provide a means for ascertaining whether the front is tilted out of a desired orientation. Such tilts are undesirable as they can cause incomplete or inefficient combustion in the retort. In general, the prior art does not provide a means of detecting both the lateral and vertical location of a flame front, the speed with which the flame front is propagating through the carbonaceous stratum and the degree to which the front deviates from a desired horizontal or vertical plane. Once these parameters of the underground flame are detected, various means can be employed to selectively speed up or hinder portions of this flame front to more efficiently effectuate the retorting process and eliminate unfavorable combustion characteristics.
A variety of methods have also been employed to control the extent, progress or uniformity of an underground flame front. One method injects an oxygen-containing gas into the formation to produce auto-oxidation of the material and then injecting a second gaseous mixture of oxygen and a combustible gas behind the front to control the speed of the flame front. This causes the combustion zone to spread vertically while the horizontal position remains substantially constant.
A second process useful in inverse-combustion involves injecting a combustion-supporting free-oxygen-gas into an underground stratum to feed the flame front and injecting a transport gas behind the combustion front to transport hydrocarbons into unburned stratum. Other methods similarly involve the injection of various gases either in front of or behind the flame front to speed up, hinder, or optimize combustion.
All of the above methods are notable in that they are applied "blind." That is, assumptions are made concerning the combustion characteristics underground and gases are introduced without specific information as to the actual conditions. This is done in the hope that the gases will achieve the desired purpose. The prior art control techniques are not employed in conjunction with a specific means of monitoring the flame front. All control methods are generalized in nature and not in response to detected anomalies or problem areas in the combustion front. In contrast, this invention provides for monitoring specific characteristics of the flame front and then the application of controlling means in direct response to detected problem areas or anomalies in the flame front. Significant advantages over the "blind" prior art control methods include: higher efficiency in locating and correcting specific unfavorable combustion characteristics; the ability to respond directly to and correct highly localized unfavorable combustion characteristics; the ability to respond more quickly as channeling, for example, occurs to prevent large scale problems; economy in usage of the control apparatus as it will be employed only where needed; and the ability to continuously monitor the flame front response to the control method, and thereby identify areas requiring continued corrective efforts.
The general object of this invention is to provide a method of controlling the progress and pattern of a combustion front of carbonaceous stratum which avoids the random or "blind" nature of prior art techniques. A more specific object of this invention is to provide a method for controlling both the vertical and lateral movement of an underground flame front. Another object of this invention is to provide a means of ascertaining the spatial orientation of the flame front and thereafter controlling the front to optimize combustion yield in a retort.